diff --git a/xembed-sni-proxy/sniproxy.cpp b/xembed-sni-proxy/sniproxy.cpp
--- a/xembed-sni-proxy/sniproxy.cpp
+++ b/xembed-sni-proxy/sniproxy.cpp
@@ -209,8 +209,9 @@
     //if the client does supports that we send directly, otherwise we'll use xtest
     auto waCookie = xcb_get_window_attributes(c, wid);
     auto windowAttributes = xcb_get_window_attributes_reply(c, waCookie, nullptr);
-    if (! (windowAttributes->all_event_masks & XCB_EVENT_MASK_BUTTON_PRESS)) {
+    if (windowAttributes && ! (windowAttributes->all_event_masks & XCB_EVENT_MASK_BUTTON_PRESS)) {
         m_injectMode = XTest;
+        free(windowAttributes);
     }
 
     //there's no damage event for the first paint, and sometimes it's not drawn immediately